It has been observed that many preparations of N.alpha.-urethane protected amino acids are contaminated with significant amounts of di- and tripeptides. In the reactions used by the prior art to synthesize these protected amino acids, the protecting group acylating reagent, 1, ##STR1## is reacted with an amino acid, 2, ##STR2## to form a N.alpha. protected amino acid, 3, ##STR3## where R is alkyl or arylalkyl and R' represents the amino acid side chain. In most circumstances, these reactions are run in aqueous/organic solvents with an inorganic base (such as sodium carbonate, magnesium oxide, etc.) to neutralize the HCl formed. The conditions, termed Schotten-Baumann, leave the carboxyl group exposed as a carboxylate anion coordinated with a metal cation, 4, ##STR4## which can undergo reaction with a second molecule of reagent 1 to form a reactive species called a mixed anhydride, 5, ##STR5## This intermediate can react with a second amino acid molecule 2 to form the N.alpha.-blocked dipeptide, 6. ##STR6##
This process can repeat to produce decreasingly smaller amounts of higher homologues (tripeptides, tetrapeptides, etc.).
The amounts of contaminating peptide impurities are dependent on a variety of factors including type of acylating agent, base, solvent, temperature, and pH. Research has been performed in order to optimize these parameters, however, all conditions found by the prior art lower but do not eliminate the impurities.
To prevent this reaction, the carboxyl group must be blocked, preferably as a labile ester. Trimethylsilyl (Tms) esters are known to be readily formed and easily hydrolyzed back to the carboxylic acid. Smithwick and Shuman [2] relied on transient trimethylsilyl protection to prepare N.sup..alpha.,N.sup.G,N.sup.G -tribenzyloxycarbonyl-L-arginine, which could previously only be formed in low yields by known routes. However, Smithwick and Shuman utilized a multistep procedure which required both the formation of lithium salts and resulted in a low yield of the final product. Theodoropoulos and co-workers, J. Org. Chem. 47, 1324 (1982), utilized Tms in the syntheses of N.alpha.-trityl amino acids. The trityl group, however, is greatly different in reactivity and size from most blocking groups. In addition, due to the chemistry of the trityl groups formation of peptide contaminants is probably not a problem. In addition the use of the Tms group in this reference is different from the present invention in that the amino group of the particular amino acid is not silylated when the desired blocking roup is attached to the amino acid (the intermediate being silylated at positions other than the N.alpha.-amino group) and this reference utilizes methanol solvolysis as opposed to aqueous hydrolysis. Presumably, the high yields obtained by this procedure were the result of the facile methanol solvolysis of the Tms ester at the end this particular synthetic route.
The prior art, however, did not recognize that contaminating peptide formation could be entirely prevented during the synthesis of N.alpha.-blocked amino acids regardless of the particular blocking group used during the synthesis procedure or that the synthesis procedure could utilize aqueous base hydrolysis conditions. It is, therefore, an object of this invention to eliminate the formation of these contaminants when producing N.alpha.-blocked amino acids.